Item is not modifiable or field is not modifiable? If the latter, it sounds like he is trying to edit the contents of a calculation field.
If he's trying to modify the layout design, the object(s) may be locked. They can be unlocked.
He is tring to modify the form from what little that I know of the program. Where would I go to see if the form is locked or unlocked?
You can't lock the layout (form), but you can lock objects that are on the layout.
If you click on an object while in layout mode, the selection "handles" (small squares at each corner of the object) will be grey instead of black if the object is locked. You can unlock the object by selecting "Unlock" from the Arrange menu.
I checked out the area that you told me and it was all greyed out. Just for my curiousity I added the user to the Power User group and this resolved the issue. I took him out of that group and set him as a standard user again and the issue returned. This would explain why it appeared like a permissions issue, but not sure why as a standard user he would be having this problem.